1. Field of the Invention
This invention relates generally to methods of making soybean plants that produce soybean seed with altered oil compositions and, more particularly, to methods where soybean seed with a mid oleic, low linolenic phenotype or soybean seed with a mid oleic, low saturate, low linolenic phenotype are produced.
2. Related Art
Plant oils are used in a variety of applications. Novel vegetable oil compositions and improved approaches to obtain oil compositions, from biosynthetic or natural plant sources, are needed. Depending upon the intended oil use, various different fatty acid compositions are desired. Plants, especially species which synthesize large amounts of oils in seeds, are an important source of oils both for edible and industrial uses. Seed oils are composed almost entirely of triacylglycerols in which fatty acids are esterified to the three hydroxyl groups of glycerol.
Soybean oil typically contains about 16-20% saturated fatty acids: 13-16% palmitate and 3-4% stearate. See generally Gunstone et al., The Lipid Handbook, Chapman & Hall, London (1994). Soybean oils have been modified by various breeding methods to create benefits for specific markets. However, a soybean oil that is broadly beneficial to major soybean oil users such as consumers of salad oil, cooking oil and frying oil, and industrial markets such as biodiesel and biolube markets, is not available. Prior soybean oils were either too expensive or lacked an important food quality property such as oxidative stability, good fried food flavor or saturated fat content, or an important biodiesel property such as appropriate nitric oxide emissions or cold tolerance or cold flow.
Higher plants synthesize fatty acids via a common metabolic pathway—the fatty acid synthetase (FAS) pathway, which is located in the plastids. β-ketoacyl-ACP synthases are important rate-limiting enzymes in the FAS of plant cells and exist in several versions. β-ketoacyl-ACP synthase I catalyzes chain elongation to palmitoyl-ACP (C16:0), whereas β-ketoacyl-ACP synthase II catalyzes chain elongation to stearoyl-ACP (C18:0). β-ketoacyl-ACP synthase IV is a variant of β-ketoacyl-ACP synthase II, and can also catalyze chain elongation to 18:0-ACP. In soybean, the major products of FAS are 16:0-ACP and 18:0-ACP. The desaturation of 18:0-ACP to form 18:1-ACP is catalyzed by a plastid-localized soluble delta-9 desaturase (also referred to as “stearoyl-ACP desaturase”). See Voelker et al., 52 Annu. Rev. Plant Physiol. Plant Mol. Biol. 335-61 (2001).
The products of the plastidial FAS and delta-9 desaturase, 16:0-ACP, 18:0-ACP, and 18:1-ACP, are hydrolyzed by specific thioesterases (FAT). Plant thioesterases can be classified into two gene families based on sequence homology and substrate preference. The first family, FATA, includes long chain acyl-ACP thioesterases having activity primarily on 18:1-ACP. Enzymes of the second family, FATB, commonly utilize 16:0-ACP (palmitoyl-ACP), 18:0-ACP (stearoyl-ACP), and 18:1-ACP (oleoyl-ACP). Such thioesterases have an important role in determining chain length during de novo fatty acid biosynthesis in plants, and thus these enzymes are useful in the provision of various modifications of fatty acyl compositions, particularly with respect to the relative proportions of various fatty acyl groups that are present in seed storage oils.
The products of the FATA and FATB reactions, the free fatty acids, leave the plastids and are converted to their respective acyl-CoA esters. Acyl-CoAs are substrates for the lipid-biosynthesis pathway (Kennedy Pathway), which is located in the endoplasmic reticulum (ER). This pathway is responsible for membrane lipid formation as well as the biosynthesis of triacylglycerols, which constitute the seed oil. In the ER there are additional membrane-bound desaturases, which can further desaturate 18:1 to polyunsaturated fatty acids. A delta-12 desaturase (FAD2) catalyzes the insertion of a double bond into 18:1 (oleic acid), forming linoleic acid (18:2). A delta-15 desaturase (FAD3) catalyzes the insertion of a double bond into 18:2, forming linolenic acid (18:3).
Inhibition of the endogenous FAD2 gene through use of transgenes that inhibit the expression of FAD2 has been shown to confer a desirable mid-oleic acid (18:1) phenotype (i.e. soybean seed comprising about 50% and 75% oleic acid by weight). Transgenes and transgenic plants that provide for inhibition of the endogenous FAD2 gene expression and a mid-oleic phenotype are disclosed in U.S. Pat. No. 7,067,722. In contrast, wild type soybean plants that lack FAD2 inhibiting transgenes typically produce seed with oleic acid compositions of less than 20%.
Soybean oil typically contains about 8% of linolenic acid (18:3) that results in reduced stability and flavor. The levels of linolenic acid (18:3) in soybean oil can be reduced by hydrogenation to improve both stability and flavor (Dutton et al., 1951; Lui and White, 1992). Unfortunately, hydrogenation results in the production of trans fatty acids, which increases the risk for coronary heart disease when consumed (Hu et al., 1997).
Conventional breeding has also been used to generate soybean lines with the linolenic levels ranging from 1%-6% (Ross et al. Crop Science, 40:383; 2000; Wilson et al. J. Oleo Sci., 50:5, 87, 2001; Wilson Lipid technology September 1999). Varieties of low linolenic acid soybean have been produced through mutation, screening and breeding (Fehr et al., 1992; Rahman and Takagi, 1997; Ross et al., 2000; Byrum et al., 1997; Stoisin et al., 1998). Certain soybean varieties with a linolenic acid content of about 1 or lower have been obtained (U.S. Pat. Nos. 5,534,425 and 5,714,670). More recently, methods for obtaining soybean plants with both low levels of linolenic acid levels as well as the yield and growth characteristics of agronomically elite soybean varieties have been disclosed (U.S. Patent Application 2006/0107348).
Oleic acid has one double bond, but is still relatively stable at high temperatures, and oils with high levels of oleic acid are suitable for cooking and other processes where heating is required. Recently, increased consumption of high oleic oils has been recommended, because oleic acid appears to lower blood levels of low density lipoproteins (“LDLs”) without affecting levels of high density lipoproteins (“HDLs”). However, some limitation of oleic acid levels is desirable, because when oleic acid is degraded at high temperatures, it creates negative flavor compounds and diminishes the positive flavors created by the oxidation of linoleic acid. Neff et al., JAOCS, 77:1303-1313 (2000); Warner et al., J. Agric. Food Chem. 49:899-905 (2001). It is thus preferable to use oils with oleic acid levels that are 65-85% or less by weight, in order to limit off-flavors in food applications such as frying oil and fried food. Other preferred oils have oleic acid levels that are greater than 55% by weight in order to improve oxidative stability.
For many oil applications, saturated fatty acid levels of less than 8% by weight or even less than about 2-3% by weight are desirable. Saturated fatty acids have high melting points which are undesirable in many applications. When used as a feedstock or fuel, saturated fatty acids cause clouding at low temperatures, and confer poor cold flow properties such as pour points and cold filter plugging points to the fuel. Oil products containing low saturated fatty acid levels may be preferred by consumers and the food industry because they are perceived as healthier and/or may be labeled as “saturated fat free” in accordance with FDA guidelines. In addition, low saturate oils reduce or eliminate the need to winterize the oil for food applications such as salad oils. In biodiesel and lubricant applications oils with low saturated fatty acid levels confer improved cold flow properties and do not cloud at low temperatures.
Soybean lines that produce seed with mid-oleic, low-linoleic acid content would be very desirable. Unfortunately, attempts to combine the mid oleic and low linolenic traits via genetic engineering approaches have been problematic. Transgenic lines where both the delta-12 desaturase (FAD2) and the delta-15 desaturase (FAD3) genes have been suppressed have seed with low linolenic levels, but the oleic acid levels are typically above the range defined for mid oleic. However, the methods disclosed here enable production of low linolenic soybean seeds that also have oleic acid levels in the mid oleic range of 55-80%. Furthermore, these methods do not entail hydrogenation processes and thus avoid the production of undesirable trans-fats.
Soybean lines that produce seed with mid-oleic, low saturate, low-linoleic acid content would be also very desirable. Methods disclosed here enable production of low linolenic soybean seeds that also have oleic acid levels in the mid oleic range of 55-80% and saturated fatty acid levels of less than 8%.